Nondestructive Inspection (NDI) determines the quality of a workpiece without causing damage to the workpiece. One NDI technique uses acoustic waves to inspect a workpiece. This technique directs an incident acoustic wave at a workpiece, senses a reflection from the workpiece and analyzes the reflection to determine the quality of the workpiece. Acoustic inspection is helpful, for example, to determine the integrity of airplane components including the wing, fuselage and empennage by detecting disbonded lap splices, corroded rivet joints and similar structural defects.
A typical apparatus for acoustically inspecting a workpiece includes a pulse generator electrically connected to a transducer assembly which generates a focused acoustic wave. The acoustic wave travels through a transmission medium and focuses onto the workpiece. Acoustic reflections from the workpiece radiate back to the transducer and causes the transducer to generate a corresponding electrical signal. A processor then analyzes the electrical signal to determine the quality of the workpiece.
Ultrasonic NDI, in particular, can improve the inspection spatial resolution and signal to noise ratio by using a focused acoustic beam. This type of NDI requires a good acoustic coupling between the transducer and the workpiece, and is most effective when applied in an immersion mode.
A known inspection apparatus uses water to serve as a coupling fluid between the transducer and the workpiece. This apparatus employs a perforated membrane between the transducer and the workpiece to retain water, reduce spillage, and leak water onto the workpiece to effect an acoustic coupling between the transducer and the workpiece. The amount of water leakage is determined by the perforation density. An example of such an apparatus is described in C. Bunyak, A Novel Acoustic Coupling Device Using Permeable Membrane, Materials Evaluation, vol. 45, p. 743 (June 1987).
In an inspection apparatus with a perforated membrane, there is no control over the amount of water leaking through the membrane's perforations. Moreover the membrane does not adequately accommodate workpiece surface variations that affect acoustic coupling with the workpiece.